The project is a continuation and an extension of long range studies in the genetic control of the immune response to the Thy-1.1 antigen. The magnitude of the immune response of mice to this antigen will be measured by the number of cells producing antibodies lytic for cells carrying the Thy-1.1 antigen, e.g., AKR thymocytes. Present studies will concentrate in the question of chromosomal localization of the Ir-Thy-1 genes determining the magnitude of the primary response to the Thy-1.1 antigen. Recently obtained data strongly suggested that at least one of the Ir-Thy-1 loci maps to the K region and may be identical with the H-2K locus. To confirm these original findings and to obtain experimental evidence of the identity of the H-2K and Ir-Thy-1 alleles, several available H-2 mutant mice and their appropriate F1 hybrids will be tested. In addition to in vivo immunization, the in vitro technique will be used, especially in experiments in which the effect of sera against H-2K and Ia antigens of responding cells will be tested. Also, breeding the D2.B6/Ir-5 congenic line will be continued as a necessary step for serological and possibly chemical characterization of the Ir-5 products.